World Library  
Flag as Inappropriate
Email this Article

Floral symmetry

Article Id: WHEBN0000709092
Reproduction Date:

Title: Floral symmetry  
Author: World Heritage Encyclopedia
Language: English
Subject: Petal, Floral diagram, Symmetry in biology, Floral formula, Glossary of plant morphology
Collection: Plant Morphology
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Floral symmetry

[Left] Normal Streptocarpus flower (zygomorphic or mirror-symmetric), and [right] peloric (radially symmetric) flower on the same plant

Floral symmetry describes whether, and how, a flower, in particular its perianth, can be divided into two or more identical or mirror-image parts.

Uncommonly, flowers may have no axis of symmetry at all, typically because their parts are spirally arranged.

Contents

  • Actinomorphic 1
  • Zygomorphic 2
  • Asymmetric 3
  • Differences 4
  • Peloria 5
  • Floral symmetry groups 6
  • See also 7
  • References 8
  • Bibliography 9

Actinomorphic

Wurmbea stricta, its tepals in actinomorphic arrangement

Most flowers are actinomorphic ("star shaped", "radial"), meaning they can be divided into 3 or more identical sectors which are related to each other by rotation about the centre of the flower. Typically, each sector might contain one tepal or one petal and one sepal and so on. It may or may not be possible to divide the flower into symmetrical halves by the same number of longitudinal planes passing through the axis: Oleander is an example of a flower without such mirror planes. Actinomorphic flowers are also called radially symmetrical or regular flowers. Other examples of actinomorphic flowers are the lily (Lilium, Liliaceae) and the buttercup (Ranunculus, Ranunculaceae).

Zygomorphic

Satyrium carneum. Ground orchid with typical zygomorphic floral anatomy

Zygomorphic ("yoke shaped", "bilateral" - from the Greek ζυγόν, zygon, yoke, and μορφή, morphe, shape) flowers can be divided by only a single plane into two mirror-image halves, much like a yoke or a person's face. Examples are orchids and the flowers of most members of the Lamiales (e.g., Scrophulariaceae and Gesneriaceae). Zygomorphic flowers generally have petals of two more different shapes, sizes, and colors.

Asymmetric

A few plant species have flowers lacking any symmetry, and therefore having a "handedness". Examples are Valeriana officinalis and Canna indica.[1]

Differences

Actinomorphic flowers are a basal angiosperm character; zygomorphic flowers are a derived character that has evolved many times.[2]

Some familiar and seemingly actinomorphic so-called flowers, such as those of daisies and dandelions (Asteraceae), and most species of Protea, are actually clusters of tiny (not necessarily actinomorphic) flowers arranged into a roughly radially symmetric inflorescence of the form known as a head, capitulum, or pseudanthium.

Peloria

Digitalis purpurea (common foxglove) displaying an aberrant peloric terminal flower and normal zygomorphic flowers

Peloria or a peloric flower refers to an aberration in which a plant that normally produces zygomorphic flowers produces actinomorphic flowers instead. This aberration can be developmental, or it can have a genetic basis: the CYCLOIDEA gene controls floral symmetry. Peloric Antirrhinum plants have been produced by knocking out this gene.[2] Many modern cultivars of Sinningia speciosa ("gloxinia") have been bred to have peloric flowers as they are larger and showier than the normally zygomorphic flowers of this species.

Charles Darwin explored peloria in Antirrhinum (snapdragon) while researching the inheritance of floral characteristics for his The Variation of Animals and Plants under Domestication.[3] Later research, using Digitalis purpurea, showed that his results[4] were largely in line with Mendelian theory.[5]

Floral symmetry groups

If we consider only those flowers which consist in a single flower, rather than a flower head or inflorescence, we can group the flowers into a relatively small number of 2D symmetry groups. Monocots are identifiable by their trimerous petals, thus monocots often have rotational symmetry of order 3. If the flower also has 3 lines of mirror symmetry the group it belongs to is the dihedral group D3. If not, then it belongs to the cyclic group C3. Eudicots with tetramerous or pentamerous petals may have rotational symmetry of order 4 or 5. Again, whether they also have mirror planes decides whether they belong to dihedral (D4 and D5) or cyclic groups (C4 or C5). The sepals of some monocot flowers develop to replicate the petals, thus, superficially, certain monocots can appear to have rotational symmetry of order 6 and belong to either symmetry group D6 or C6. It must be remembered however, that flower symmetry is rarely perfect in the way that geometric symmetry is. The general layout of a flower belongs to the above symmetry groups, but an individual flower will not show exact symmetry.

See also

References

  1. ^ Weberling, Focko (1992). Morphology of Flowers and Inflorescences. Cambridge University Press. p. 19.  
  2. ^ a b Losos, J.B., Mason, K.A, Singer, S.R. Biology (8th ed.). New York: McGraw Hill. 
  3. ^ Darwin 1868, pp. 33–34
  4. ^ Darwin 1868, p. 46
  5. ^ Keeble, Frederick, Pellew, C, Jones, WN (1910). "The Inheritance of Peloria and Flower-Colour in Foxgloves (Digitalis purpurea)". New Phytologist 9.  

Bibliography

  • Endress PK (February 2001). "Evolution of floral symmetry". Curr. Opin. Plant Biol. 4 (1): 86–91.  
  • Neal P.R., Dafni A., Giurfa M. (1998). "Floral symmetry and its role in plant-pollinator systems: terminology, distribution, and hypotheses". Annu Rev Ecol Syst 29: 345–373.  
  • Darwin, Charles (1868). The variation of animals and plants under domestication II. London: John Murray. 
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.